Here's a sample from the SQL Server log:

Database PRD: IO is frozen for snapshot

Database PRD: IO is thawed

Database backed up: Database: PRD, creation date(time): 2003/07/26(16:43:42), pages dumped: 1, first LSN:...

I have been looking at this and the solution is the Virtual Backup Device API. It is possible to create a program that will issue freeze and thaw commands...